Search PubMed⌕ Search

Biomedical subjects

W L Pak

Publications and source records attributed to W L Pak.

At least 73 records · Page 4Linked to original sources
PubMed1975

Intracellular recordings of rod responses during dark-adaptation.

1. Dark-adaptation of rod photoreceptors has been studied in the isolated axolotl (Ambystoma mexicanum) retina by intracellular recordings. Rod responsiveness was greatly reduced immediately after a 30 sec partial bleach, but partially recovered with time in the dark. 2. In parallel spectrophotometric measurements using isolated retinas, regeneration of the rod pigment could not be detected after a 30 sec bleach. 3. During rod dark-adaptation, the response of a rod to a given stimulus increased in amplitude, duration, and rate of rise but did not recover completely to the dark-adapted values. Response latency was lengthened immediately after a bleach but ultimately returned to the dark-adapted level. 4. The time courses of dark-adaptation determined on the basis of the intensity of a stimulus needed to evoke a response having a criterion amplitude, a criterion duration, or a criterion rate of rise were similar. On the other hand changes in latency of the response and magnitude of the saturated amplitude followed different time courses. Change in log threshold was found to be related to change in saturated amplitude by an exponential function during dark-adaptation. 5. After bleaching 10% or less of the rod pigment, the kinetics of both recovery of log threshold and decrease in absorbance at 400 nm (metarhodopsin II+free retinal) could be described by two concurrent first-order processes having similar time constants. However, after bleaching more than 10% of the rod pigment, changes in sensitivity and absorbance did not follow parallel time courses. 6. Metarhodopsin III cannot be solely responsible for setting the axolotl rod sensitivity since rod thresholds decrease monotonically during dark-adaptation whereas meta III concentration reaches a peak 3 min after the bleach and decreases thereafter.

Ambystoma↗
PubMed1974

Fast electrical potential from a long-lived, long-wavelength photoproduct of fly visual pigment.

A rapid electrical potential, which we have named the M-potential, can be obtained from the Drosophila eye using a high energy flash stimulus. The potential can be elicited from the normal fly, but it is especially prominent in the mutant norp A(P12) (a phototransduction mutant), particularly if the eye color pigments are genetically removed from the eye. Several lines of evidence suggest that the M-potential arises from photoexcitation of long-lived metarhodopsin. Photoexcitation of rhodopsin does not produce a comparable potential. The spectral sensitivity of the M-potential peaks at about 575 nm. The M-potential pigment (metarhodopsin) can be shown to photoconvert back and forth with a "silent pigment(s)" absorbing maximally at about 485 nm. The silent pigment presumably is rhodopsin. These results support the recent spectrophotometric findings that dipteran metarhodopsin absorbs at much longer wavelengths than rhodopsin. The M-potential probably is related to the photoproduct component of the early receptor potential (ERP). Two major differences between the M-potential and the classical ERP are: (a) Drosophila rhodopsin does not produce a rapid photoresponse, and (b) an anesthetized or freshly sacrificed animal does not yield the M-potential. As in the case of the ERP, the M-potential appears to be a response associated with a particular state of the fly visual pigment. Therefore, it should be useful in in vivo investigations of the fly visual pigment, about which little is known.

Animals↗
PubMed1974

Light-induced changes in photoreceptor membrane resistance and potential in Gecko retinas. I. Preparations treated to reduce lateral interactions.

The time-course of the light-induced changes in membrane voltage and resistance were measured for single photoreceptors in the retina of Gekko gekko. In the surgically isolated retina, small stimuli directed toward the impaled receptor produced a membrane hyperpolarization the time-course of which was identical to that of the increase in membrane resistance. In the eyecup preparation nearly identical time-courses were evoked only after perfusion of the vitreous surface with solution having high (Mg(++)). Disparate time-courses were obtained in (a) the isolated retina when large or displaced stimuli were used, and (b) the eyecup preparation when it was treated normally (see Pinto and Pak. 1974. J. Gen. Physiol. 64:49) and when it was exposed to aspartate ions or hypoxia. These results are consistent with the hypothesis that the receptor potential (elicited in the impaled receptor as a result of quanta only it captures) is generated by a single ionic process that decreases membrane conductance. These measurements provide a means to distinguish the receptor potential from interactions. From direct measurements of membrane time constant and total resistance in darkness, total membrane capacitance was calculated. The mean capacitance was 7.1 x 10(-5) microF. This high value is consistent with anatomical observations of membrane infoldings at the base of gecko photoreceptors.

Animals↗
PubMed1974

Light-induced changes in photoreceptor membrane resistance and potential in Gecko retinas. II. Preparations with active lateral interactions.

The time-course of light-induced changes in membrane voltage and resistance were measured in single photoreceptors in eyecup preparations of Gekko gekko. A small circular stimulus directed toward the impaled receptor produced membrane hyperpolarization. Application of a steady annular light to the receptor periphery resulted in diminution of the receptor's response to the stimulus. The effects of illumination of the surrounding receptors were isolated by directing a small, steady desensitizing light to the impaled receptor and then applying a peripheral stimulus. Brief stimuli produced a transient decrease in resistance with rapid onset and offset, a time-course similar to that of the response diminution. For some cells a depolarization that coincided with the resistance decrease was seen. During illumination with prolonged stimuli the resistance decrease was followed by a slow increase. After offset resistance rose transiently above the original value and then returned slowly to its original value. The slow resistance changes were not accompanied by changes in membrane voltage. The response diminution, resistance decrease, and depolarization were not observed in retinas treated with aspartate or hypoxia. It is therefore concluded that these effects are mediated by horizontal cells. The diminution is achieved by shunting the receptor potential and may play a role in field adaptation.

Adaptation, Ocular↗
PubMed1972

Adaptation in retinal rods of axolotl: intracellular recordings.

Intracellular recordings of the late receptor potential from rods of isolated axolotl retinas revealed the existence of a dark adaptation mechanism that is independent of rod pigment regeneration. Response amplitude of individual rods was measured as a function of intensity both before and at various times after exposure to bleaching illumination. The rod sensitivity increased by at least 3 to 4 log units during a period of 15 to 25 minutes following the bleach. During this time rod pigment regeneration was either too small to be measured or was nonexistent in our preparation.

Ambystoma↗
PubMed1971

On-transient of insect electroretinogram: its cellular origin.

Electrical responses to light stimuli were recorded intracellularly from the retinula cells and the laminar cells of the wild type and the x-7 mutant of Drosophila melanogaster in order to determine the cellular origin of the on-transient of the insect electroretinogram. The response of the retinula cell of the mutant x-7 closely resembles that of the wild type even though the ontranstent is absent from the electroretinogram of x-7. Neither contains a spike-like component that can be identified with the on-transient of the electroretinogram. However, a spike-like response resembling the on-transient in latency and time course has been obtained from one of the several types of cells in the lamina of the wild type. Moreover, the same response could not be obtained from the laminar cells of the mutant x-7. Thus, the on-transient does not appear to originate from the retinula cell but probably from one or more of several types of cells in the lamina.

Animals↗
PubMed1967

Rhodopsin: responses from transient intermediates formed during its bleaching.

Fast electrical responses elicited from the transient intermediates in the bleaching of rhodopsin have been investigated in the retina of the albino rat. In the experiments we used two-flash stimuli triggered sequentially with a continuously variable time delay between them. At room temperature the potential is biphasic in wave-form. The dominant, corneal-positive component is preceded by a small, corneal-negative component. Cooling the retina to about 0 degrees C suppresses the positive component and isolates the faster, negative component. Experiments with color filters show that these two components display different spectral sensitivities and, hence, suggest that each of them is produced by a different photoproduct of bleaching.

Animals↗